Antenna feed



' i 1111/1/11]: ELECTRIC FIELD-E PLANE J June 30, 1959 J. s. ARNOLD ETAL 2,893,003

ANTENNA FEED Filed June 26, 1957 -11 Sheets-Sheet 1 I 1 J F/GZ aIII/III!IIIIIIIIIIIIIIIIII/IIIIIIII IIIIIIIIIIIIIIIIIIIIIIIII,

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JAMES S. ARNOLD RALPH n. DRESSEL HERBERT n. HAAS INVENTORS ATTORNEYSJune 30, 1959 J. 5. ARNOLD ET AL ANTENNA FEED 11 Sheets-Sheet 2 FiledJune 26, 1957 FIG. 4.

E PLAN 5 PLANIE 60 8Q ANGLE FROM AXIS-DEGREES JAMES $.AR/VOLD ANGLE FROMAXIS- DEGREES RALPH W DRESSEL FIG. 5. m H445.

HERBERT INVENTORS BY 05W June 30, 1959 J. 5. ARNOLD ET AL 2,

ANTENNAFEED Filed June 26, 1957 11 Sheets-Sheet a POLARIZATION JAMES S.ARNO RALPH W DRE'S L HERBERTW HAAS' INVENTORS BY @i;

ATTORNEYS 111M 1959 J. s. ARNOLD ET AL 2,893,003

ANTENNA FEED ll Sheets-Sheet 4 Filed June 26, 1957 'E PLANE ----H PLANEDEGREES 5 m mJUEOUO IFGZUEPm ANGLE FROM AXIS wfim wmmm MW RR AD $.5 MM%w ATTORNEYS J. S. ARNOLD ET AL June 30, 1959 ANTENNA FEED 11Sheets-Shet 5 Filed June 26, 1957 FIGS/3 5 m m m ANGLE FROM AXIS-DEGREFS mJmmawo zhwzmmhm ANGLE FROM AXIS-DEGREES L m wx OS W NM m mamSWW. wm Mam MRH IIIIIIl'IIIA IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII,,,,,,,,,,,,,,,,,,,,,,,,.,,,,.W.

BY ,glflflan FIG. 12.

RELATIYE FIELD STRENGTH DECI BELS RELATIVE FIELD STRENGTH DECIBELS June30, 1959 Filed June 26, 1957 ANTEINN FEED ll Sheets-Sheet 6 W Y \Y\ V/'\\V 20 4o Vo 80 I00 I20 I40 I60 ueo ANGLE FROM AXIS-DEGREE 0 .ZA &Lil.

v q .BA f L H PLANE \0 IX //%d/ b J f R 2o 40 so so I00 I I I I ANGLEFROM AXIS-DEGREES INVENTORS FIG. /6. JAMES s. ARNOLD RALPH W DRESSEL HEREH7 9!. H445 M ATTORNEYS June 30, 1959 J. s. ARNOLD ET AL ANTENNA FEEDll Sheets-Sheet '7 Filed June 26, 1957 E PLANE 4o so ANGLE FROM AXISDEGREES m m m omo-x 5zmEw 3!. mwi um H PLANE mimawai gzmmhw 3!. was]?INVENTORS' ANGLE FROM AXIS DEGREES L m. DQEJA I- A O OfiH W A R w T Hm MBW W E mm w F June 30, 1959 J. 5. ARNOLD ET AL 2,

ANTENNA FEED Filed June 26, 1957 ll Sheets-Sheet 8 III III!

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' s F/ .2 EQUIPHIASE CONTOURS JAMES a g v olz RALPH W. DRESSEL H RBERTW.HAAS BY 00% RELATIVE FIELD STRENGTH DECIBELS June 30, 959 J. s.ARNOLD'ET AL 2,393,003

I ANTENNA FEED I Filed June 26, v 1957 -l1 Sheets-Sheet 9 FIG-22 E PLANERELATIVE FIELD STRENGTH DEGIBELS N 60 80 I I ANGLE FROM AXIS-DEGREES 6O'80 I00 I20 I40 I I ANGLE FROM AXIS DEGREES FIG. 23.

JAMES 5. ARNOLD RALPH M. DRESSEL HERBERT I4. HAAS INVENTORS ATTORNEYSRELATIVE FIELD STRENGTH-DEOIBELS June 30, 1959 I J. 5. ARNOLD ET AL2,893,003

' ANTENNA FEED Filed Jun 26, 1957 I 11 Sheets-Sheet 10 FIG. 25.

E PLANE 20 40 V 60 80 I00 I20 I40 I60 I80 ANGLE FROM AXIS DEGREES H PLAE RELATIVE FIELD STRENGTH-DECIBELS O 20 4O 6O 80 I00 I20 I40 I I ANGLEFROM AXIS-DEGREES FIG. 26. 4a 43 JAMES S. ARNOLD 45- 4/ RALPH W. DRESSELHERBERT n. HAAS K INVENTORS ATTORNEYS June 30, 1959 Filed June 26, 1957RELATIYE FIELD STRENGTH 'DECIBELS u I I "-IO l l I RELATIVE FIELDSTRENGTH DEGIBELS J. s. ARNOLD ET AL 2,893,003

ANTENNAFEEID ll Sheets-Sheet 11 -a'o 2 o -16 b +l o +2'o 1S8 ANGLE FROMAXIS- DEGREES E PLANE, EXTREME RIGHT I ------EPLANE, EXTREME LEFT PLANEMES. 5, ARNOLD RALPH u. DRESSEL HERBERT m H443 INVENTORS ANGLE FROMAXIS-DEGREES ATTORNEYS .eter extending along the axis of the feed.

front in the aperture.

United states Patent ANTENNA FEED James S. Arnold, Stanford, Calif., andRalph W. Dressel,

Las Cruces, and Herbert W. Haas, State College,

N. Mex., assignors to the United States of America as represented by theSecretary of the Navy Application June 26, 1957, Serial No. 668,273Claims. (Cl. 343-781) The present invention relates to microwave'antennafeeds. More particularly, it relates to an antenna feed of "a typeparticularly suited for use with parabolic dish reflectors.

a A common form of antenna used at centimeter wavelengths consists of aparabolic reflector illuminated by a radio frequency power sourcelocated at thereflector focus. The power source is frequently referredto as the'antenna feed. The basic function of the feed is to extractpower from a radio frequency transmission line and direct it toward thereflector as an electromagnetic field. The properties of the secondaryradiation field generated by the subsequent reflection of energy fromthe paraboloid are directly determined by the space distribution, thepolarization, and the phase pattern of the primary radiation from thefeed. Close control over the primary field is therefore'necessary inorder to produce a secondary field having predetermined characteristics.

The use of a parabolic reflector as part of an antenna system placesvery specific requirements upon the space configuration of the primaryfield established by the feed.

It is frequently desired that. the distant field be plane polarized. Theradiation from the feed, in that case, must be so polarized that afterreflection from the paraboloid theresulting electric field vectors areall parallel. It may be demonstrated that the required primary field beof such form that lines describing the direction of the electric vectorform circles on the surface of a sphere whose center is located at thefeed head. These circles ..are defined by the intersection of the spherewith a set of vertical planes passing through the end of the diam- It isfurther known that the equiphase surfaces in the primary field must bespherical.

In principle, it is possible to design a reflector to match anyarbitrary phase surface supplied by a feed so that the phase surfacewill be transformed into a plane phase However, the diflicultiesinvolved in obtaining phase measurements and the accuracy with which thephase must be known before an appropriate reflector contour can becalculated render the design .diflicult. It is more practical thereforeto construct the feed to supply the required spherical phase front forcontrol of a three dimensional field, proper terminations could notalways be obtained.

It is desirable that the feed have no preferential axis of polarization.The feed may then be rotated on its axes without modifying the wave formor the polarization of the microwave power passing through it. Suchfeeds are termed rotationally symmetrical feeds.

Among the known rotationally symmetrical feeds is the 2,893,003 PatentedJune 1959 ICC direct radiating type comprising, for example, a hornplaced in front of the reflector. A horn feed is highly effective, butthe waveguide carrying power to the feed, together with supportingstructure, must pass in front of the aperture of the reflector therecreating interference inbthe secondary field and scattering energy intothe side The rear feed is another of theknown symmetrical types. Therear feed comprises a waveguide terminated by a reflecting disk whichserves to direct the radiation back along the outside of the waveguide.A rear feed may be inserted through the center of the reflector leavingall of the support and the mechanism for rotation behind the reflectingsurface. As a' result, interference with the secondary field in thereflector aperture is minimized.

Cutler earlier constructed a geometrically symmetrical rear feedcomprising a short section of circular waveguide terminated by a flatdisk of metal. The dimensions of the disk and the spacing from the endof the waveguide were experimentally chosen to give an optimum .powertransfer from the waveguide. However, asthe dimensions of the diskwere'of the order of a wavelength, the disk became strongly excited bythe radiation issuing "from the end of the waveguide. It istherefore-more'approprr ate to describe the disk as a secondary radiatorrather than as a simple reflector.

One consequence of the excitation of Cutlers disk is that the equiphasesurfaces of the reflected radiation are toroidal rather than sphericalas required in an, ideal point source. The Cutler rear feed hastherefore been occasionally referred to as a ring focus feed.,Rotationally symmetrical radiation patterns are not obtained with aring focus feed, nor is the secondary-fieldplari'e polarized. a i

Accordingly, it is an object of'the present invention to .provide in amicrowave antenna a rotationally symmetrical rear feed for exciting aparabolic reflector in such a manner that the distant field will possessthe characteris- Another object of the present invention is'to provide arotationally symmetrical rear feed for a parabolic'reflector in whichback radiation from the feed isminimized.

A further object of the present invention is to provide a rear feedexhibiting the properties of a resonant cavity and thereby affordinggreater control over the resulting pattern of radiation.

An additional object of the present invention -is to provide an antennafeed providing modulation of the secondary field by rotation of theantenna feed.

Still another object of the present invention is to provide an antennafeed Well balanced mechanically to permit high speed rotation of thefeed about its longitudina axis for modulating the secondary field.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings, wherein:

Fig. l is an axial section of one embodiment of the feed of the presentinvention illustrating the parameters which may be varied to control theprimary radiation pattern;

Fig. 2 schematically represents the feed of Fig. l and illustrates thespatial electric field for a fourth ordercurrent distribution;

Fig. 3 is a perspective view of the feed of Fig. 1 illustrating thecharge and current distribution on the outer surface of the feed for afourth order charge distribution;

Fig. 4 is a chart which illustrates radiation patterns ob- 3 tained withthe feed of Fig. 1 operating in the TE mode for various values of capdiameter y;

Fig. 5 is a chart illustrating radiation patterns ob tained withtheieedof Fig. 1 operating in the TE mode for various values of cap side lengthx;

'Figs. .6 and 7 are schematics which illustrate the electricpolarization of the radiation from the feed of Fig. 1 operated in the TEmode with fourth order charge distribution;

Fig. 8 illustrates the spatial electric field of the feed ofFig. loperated in the TM mode with sixth order charge distribution;

Fig. 9 is a chart which illustrates the radiation pattern of the feedshown in Fig. 8;

Figs. 10 'and 11 illustrate the polarization of the electric field ofthe feed of Fig. 8;

- Fig.-" 1-2 is an axial section of a second embodiment of th'e'presentinvention;

Figs. 13 and 14 are charts illustrating the E andH plane radiationpatterns, respectively, of the feed of Fig. 12 forvarious values of capside length x;

f-Figs. 1-5 and 16 are charts showing the E and H plane radiationpatterns, respectively, of the feed of Fig. 12 for'var'ious insertionlengths Z with a cap side length equal to one wavelength;

Figs. 17 and 18 are pattern charts similar to those of Figs. and 16except that the side length is equal to "056 wavelength;

Figs. 19 and 20 are schematics which illustrate equiphase contours inthe E and H planes, respectively, from a feedsinjilar to that of Fig. 12and having a side length 'of ,016 wavelength;

' Fig. 21 is an axial section of a modification of the 'feedof Fig. 12;

'Figs. 22 and 23 are E and H plane radiation pattern charts,respectively, of the feed of Fig. 21 for various values of insertionlength Z;

Fig. 24 is an axial section of a further modification of the feedillustrated in Fig. 21;

" Figs. 25 and 26 are radiation pattern charts in the E andHplanes,respectively, of the feed of Fig. 24 for various values of spacerdiameter k;

Fig. 27 is an axial section of a feed similar to that of Fig. 24,illustrating details of its construction;

Fig. 28 is a distant field radiation pattern obtained with .-faceis.referred to as the secondary pattern.

Cap 33 is ofcylindrical shape and is formed ofan end disk 34, having adiameter of critical dimension y, secured. to a circular side wall 35having a critical length x. Cap 33 is supported coaxially with waveguide32 by adielectric rod36 inserted therein and secured to the.ce'nt'en'ofdisk34 by any suitable means. Rod 36 also enablesthediameter of waveguide 32 to be reduced withldut' encountering cutoff.The cap dimensions x and y are selected so as to provide a resonantcavity terminationof waveguide 32. The depth Z of insertion of waveguide32 into cap 33 is critical and afiords one means of ,controlling theprimary radiation pattern, as

described more sp ecific'ally hereinafter with reference to "anotherembodiment of the invention.

For matching purposes, however, it is desirable that the depthZ be"'equal to a quarter-wavelength.

Because of the large radiating aperture 37 and because of the small sizeof the cap 33, little energy is actually stored within the resonantcavity of the feed. Nevertheless, there is a resonant mode orcombination of modes that serve to transform the TE mode of the circularwaveguide into a wave progressing back toward the reflector along theoutside of the Waveguide. The resonant modes within the cavity arecontrolled by choice of the variables x, y, and z to achieve the desiredradiation patterns.

The electromagnetic field surrounding thefeed of Fig. l is a function ofthe resonantmode within cap 33 as well as the surface currents on theoutside of the cap and the exterior of waveguide 32. Fora rotationallysymmetrical cap, two different modes can be excited by the principal orTE mode in the waveguide 32. The existence of one or the other modedepends upon the diameter y of cap 33.

Fig. '3 illustrates the field distribution within the cap 33. It will beseenthat the field distribution is quite complex andrepresents atransition from.the TE circular waveguide mode of the TE coaxial mode bymeans of the resonant cavity within cap 33. The electric fielddistribution is illustrated at only one instant of time. As timeadvances the waves will also advance through space in such a manner thatthe envelope tends toward a sphere. However, close to the 'feed cap, itis apparent that the envelope must depart significantly from a sphericalshape.

From Fig. 2 it is apparent that the surface currents circulating overthe outside of the cap play an important part in determining the spacedistribution of the field. Standing waves of current and charge areexcited by the radiation passing over the edge of the cap and are anintegral part of the total radiating system. Fig. 3 further illustratesthe charge and current distribution on the surface of cap 33. Since thecap is symmetrically excited, all of the charge antinodes occur in the Eplane.

Depending upon'the dimensions of the cap, the current and chargedistribution over its surface may vary. The distributions can beclassified according to the number of charge antinodes appearing on theouter surface of the cap. The minimum-number of charge antinodes thatmay occur is two since the dipole is the lowest order radiator. Only theeven orders of antinodes may appear as odd orders are excluded by theform of cap excitation. By varying the diameter y and side length x ofthe cap, one order oranother of charge distribution may be enhanced. Ifthe dimensionsof the cap are intermediate between those characteristicto two adjacent charge distribution orders, a mixture of orders occursand the resultant radiation field isa superpositionof the fieldsassociated with each order.

Fig. 4 illustratesthe primary, field distribution for various capdiameters 'y with the length x of side 35 held constant. The dimensionalunits are the wavelength of field distribution of Fig. 2. As thediameter y of cap 33 is decreased, the radiation from disk 34 becomesstronger indicating an approach to the characteristic dimensions for thefourth order current'distribution. The resulting influence uponthe'jprirriary' lobe in the E plane is marked but very little changeoccursin the primary lobe for the H plane. i

Fig. 5 illustrates, the, radiationpatterns resulting from varying thelength x'of'side 35with the diameter y of disk 35 constant at 1.2wavelengths. The primary lobes in both the Band H planesremainrelatively unaffected. However,the radiation from theback of thefeed changes from a lobe. to a null at the 180 position. The range ofvariation of the sideQIength x lies intermediate'io the characteristicdimensions'jfor .currenfdist'ributions 05 orders 2 and -4. Sinceftherelative strengthof the normal modes is determined byx, at a particularvalue of x, i.e.

35am contributions from eachof themodes are at 1801 relative phase andare equal, resulting in complete cancellation. v I Figs. 6 and- 7illustrate the space distribution of electric-polarization for the feedof Fig. 1 operated in the TE mode and having a fourth order surfacecurrent distribution. It will be seen that the'feed of Fig. 1 does notproduce ideal polarization for all parabolic reflectors. However, thepolarization is satisfactory for reflectors of limited aperture.

The radiation patterns of Figs. 4 and 5 and the polarization diagrams ofFigs.r6 and 7 concern operation of the feed in the TE mode. The cut-off,wavelength for the TE mode is expressed by the approximate formula E2(b--a), TM mode For. agiven operating frequency, the difference betweenthe diam eter'of waveguide 32 and the diameter of cap 33 must be atleastone-half a wavelengthto support the TM 'mode. l

vA complete change in the structure of the radiation field aboutthe feedtakes place when the TM mode is dominant within the resonant cap 33.Fig. 8 illustrates the electric field distribution in the E plane. Fig.8 refers to an instant of time when the electric field is a maximumwithin the cap cavity. .The field structure is modifiedbothby the changein mode inside cap 33 and by the change in the current distribution overthe exterior. The dimensions are sufliciently large to support asixthorder current distribution causing six nulls in the radiation fromdisk 34 and two nulls in the radiation from aperture 37.

.Fig. 9 illustrates the radiation pattern obtained by operating in theTM mode- The radiation from disk 34 is nearly as strong as thatfromaperture 37. Figs. 1 0 and 11 illustrate the polarization of radiationfrom the feed of Fig. 1 operated in the TM mode. The angle subtended atthe feed by the reflector must be less than 30 to provide planepolarization of the distant field.

- Neither the pure TE nor TM modes provide symmetry in the E and H planeradiation patterns. For the TE mode, the E plane pattern is broader thanthe H plane. It is necessary that the primary radiation intensity becircularly symmetrical to obtain symmetry in the distant field. Symmetrymay be provided by appropriately mixing the radiation fields associatedwith the two charaeteristic modes in the cap. I nFig. 12, a foldedconeresonant cap feed is illustrated'which permits adjustment of therelative intensity of the TE and TM modes therein. The resonant cap.4-1comprises a-conical end plate 42 secured at its apexto a dielectric rod36 inserted into the circular waveguide 32 which conveys power forradiation from the transmitter (not shown). End plate 42 is arrangedcoaxially with waveguide 32. A sidewall 43 extending rearwardly towardthe reflector (not shown) and having a length x is joinedperpendicularlyto the periphery of endplate 42-thus assuming the shapeof a truncated cone. The side length of conical end plate 42 is somewhatgreaterthan one half wavelength (from 0.6)\ to 0.7)) and-thefincludedangle at the vertex thereof is substantially 160. The length x of side43 is variable, as is the 'de'pthZ of the insertion of waveguide 32 intotheresonant cap 41 to control the radiation pattern of the feed. Adotted line 44 marks the distance of one half wavelength from the apexof end plate 42.- That portion of 6 TM mode. 'That portion of the feedto the leftline- 44 is sufliciently large to support both modes. Thus,the distance z of insertion of waveguide 32 into cap 41 controls theattenuation of the TM mode and hence the relative mixture of the twocharacteristic modes.

In order that the TM mode be cutoff, the length x of side 43 must be asubstantial part of a wavelength. As the length of side 43 is made lessthan a wavelength, cut-. off dimensions calculated on the basis ofinfinitely longer waveguides no longer apply. Instead, the T M mode isgradually released in spite of the fact that the dimensions of end plate42 appear too small to support it. Figs. 13 and 14 illustrate radiationpattern obtained with an insertion Z of 0.2), a value ordinarilysulficient to guarantee cutoff for the TM mode. With a constant capdiameter of 0.68)., a variation in side length x from 1.0) to 0.3).shows the release of the TM mode. For a side length of x=0.6)treasonably good symmetry between the E and H plane patterns are obtainedthrough the angular interval of 0 to 50.

Figs. 15 and 16 illustrate radiation patterns obtained by varyinginsertion Z of waveguide 32 into cap 41. The side length x is equal toone wavelength. Small values of z yield a radiation pattern typical ofthe TE mode, since the dimensions are beyond cutoff for the TM mode.However, as z is increased, the E plane pattern grows progressivelynarrower and as z passes the critical value of 0.5) there is a rapidchange in pattern structure. For all values of z greater than 0.67\, theratiation pattern is typical of' the TM mode. A progressive change withincreasing z occurs in the H plane, but the patterns fold overthemselves so that the total variation is small. By interpolationbetween the curves of Figs. 15 and 16, a setting of z=0.42)\ is obtainedas the optimum for circular symmetry.

Figs. 17 and 18 illustrate the radiation patternsobtained with a feedhaving a side length x of only a fraction of a wavelength. Increasingthe value of 2 no longer narrows the primary pattern in the E plane asin Fig. 15, since the TM mode is present for all values of z. Instead,both the E plane and H plane primary patterns grow in angular width withincreasing z.

Figs. 19 and 20 illustrate the equiphase contours of the radiation inthe E and H planes respectively from the feed of Fig. 12 having a sidelength x of 0.6)., a cone side length of 0.68). for plate 42, and awaveguide insertion Z of 0.2). The white curves mark the loci ofconstant phase. The curvature of the equiphase contours in the E and Hplanes is not identical, and therefore the ideal reflector is not aparaboloid of revolution. However, if the reflector possesses areasonably long focal length (about 10). or greater) satisfactoryoperation will be obtained. If the focal length is short, say of theorder of 6a or less, a paraboloid of revolution produces serious phaseerrors in the resultant aperture field.

Fig. 21 illustrates a feed similar to the feed of Fig. 12 except thatthe resonant cavity within' the cap 41 is filled with a low lossdielectric material 45. The dielectrio material 45 supports the metalliccap 41 and accurately maintains its position with respect to thecircular waveguide 32. The wavelength of the radiation passing throughthe dielectric medium is reduced according to the index of refraction ofthe dielectric. As a result, the cutoff of the TM mode is altered andthe effective dimensions of cap 41 are increased. As the radius of thewaveguide is fixed, it is inappropriate merely to alter dimensionsproportionately with the index of refraction to preserve the desiredbalance between the TM and TE modes. It is necessary to reduce the coneside length of plate 42 in accordance with thenew cutofi dimensionsspecified in the following formula:

TM mode- E feed to the right ofline 44 is beyond cutofi for the 7 wherea and b are the outside radius of the waveguide 7 and the inside radiusof the cap, respectively; is the velocity of lightin vfree space, v isthe frequency of the radiation ands is the dielectric constant of thefillin-g material.

Figs. 22 and 23 illustrate radiation patterns obtained with the feed ofFig. 21 for various values of waveguide insertion Z. Due to thereduction in cap diameter and to the refraction of the radiation at thedielectric-air interface, the radiation patterns are broadened ascompared with those of Figs. 17 and 18. The E and H plane patternsdemonstrate that reasonable symmetry will be obtained for small valuesof z. Further improvement in circular symmetry can be achieved byshaping the dielectric in the aperture of the feed.

The primary radiation patterns for the feeds of Figs. 1, 12, and 21 showthe presenceof undesired radiation from the sides and back of the cap.Radiation from the back of the feed must be suppressed before low sidelobe levels can be realized in the secondary pattern. In Fig. 24, doublecap feed is illustrated which not only reduces the radiation from theback of the feed, but also improves the circular symmetry of the primaryradiation pattern.

The double cap feed comprises a feed of the type of Fig. 21 combinedwith an outer cap 46 similar to cap 41 but spaced apart therefrom by aconductive spacer 47 having a diameter 'k. The sides 48 of cap 46preferably extend beyond the sides of inner cap 41. A resonant cavity 49is thus formed in the space separating caps 41 and 46, the length ofwhich is adjustable by varying the diameter k of spacer 47. If desired,the cavity 49 may be'filled with solid dielectric material to furtheralter the characteristics of the radiation pattern.

Figs. 25 and 26 demonstrate the effect of varying the length of cavity49 by means of increasing the diameter k of spacer 47. The initial valueof spacer diameter k is 0.2)., where A is the free space Wavelength ofthe radiation.

As k is increased, the field at the edge of cap 46 progressively changesfromelectric to magnetic and back again to electric according to whetherthe number of quarter wavelengths included within the cavity 49 is oddor even. For values of k in the vicinity of 0.20). the length of cavity49 is equal to approximately one mode wavelength and the field at theedge is purely magnetic. The primary pattern is nearly the same as thatfor a single cap feed except that the symmetry between the E and Hplanes is more pronounced and the radiation from the back of the outercap is less. The interaction of the electric field of cavity 49 with thefield from the aperture of cap 41 is apparent in the E plane patternsfor larger values of k.. As the value of k passes through the criticalresonance dimensions, reinforcement rapidly changes to partialcancellation. The deeper the cavity the more pronounced is theresonance.

Fig. 27 illustrates a modification of the feed of Fig. 24 in which theresonant cavity 49 is filled with solid dielectric material. Inner cap41 and outer cap 46 are secured to dielectric rod 36 by a screw 51, thespacer 47 being constituted by a boss turned on the outer surface of cap41. The dielectric material 45 filling the cavity of inner cap 41 isheld in place by a pin 52 of similar dielectric material passedtransversely through rod 36. Lower edge 53 of the dielectric filling ofcavity 49 is tapered approximately at 20 to the vertical to intersectthe lower lip of side 43 at an angle of approximately The lower lip 54of the dielectric filling of cavity 49 intersects edge 53 at an angle ofapproximately 60 to the vertical.

' Polystyrene may suitably be used as the dielectric filling of cavity49 while Teflon may suitably be used to fill the inner cavity 45.

Fig. 29 illustrates the secondary pattern of the feed of Fig. 27combined with a reflector inches in diameter.-.and havinga focal-lengthof 8 inches. Circular symmetry of the pattern of Fig. 29 is demonstratedby the fact that every cross section plane through the beam is similarwithin 1 /2 db. In addition, the radiation within the main lobe is verynearly plane polarized. Measurements of the cross components ofpolarization indicate that these are less than --30 db with respect tothe peak of the main lobe.

-While the double cap feed of Figs. 24 and 27 provides excellentcircular symmetry, it is also possible by means of the feed to create anasymmetrical field pattern. As previously demonstrated, spacer 47 exertsa strong influence on the distribution of primary radiation from thefeed. If spacer 47 is placed eccentrically between caps 41 and 46 asasymmetrical primary pattern will result. For a given eccentricity, thegreatest asymmetry will, of course, be obtained when the mean radius ofspacer 47 corresponds to the resonance dimensions of cavity 49. A spacerhaving a radius of 0.31% placed ofi center by 0.02 yields the maximumasymmetry. Such a minor eccentricity does not add serious mechanicalunbalance to the feed so that the feed may be rotated on axis at veryhigh speed to produce a high frequency modulation of the radiationfield.

As shown in Fig. 29, rotation of a feed containing an eccentricallyplaced spacer produces modulation solely in the "E plane, provided thefeed is set at the reflector focus. The degree of modulation in the Eplane is determined not only by the asymmetry of the primary pattern butalso by the focal length and diameter of the reflector. Thus there areavailable ,a large number of parameters for adjusting both the fieldpattern and the modulation obtainable with a double cap feed having aneccentrically placed spacer.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmaybe practiced otherwise than as specifically described.

What is claimed is:

1. An antenna feed for controlling the emission of energy from the endof a waveguide, comprising a cap having a conical base and a side walljoined to said base, said side wall being in the form of a truncatedcone, said base and side wall being joined at the smaller diameter ofsaid side wall so as to enclose the apex of said conical base, and meansfor securing said cap with the apex of the base thereof spaced axiallyfrom the open end of the waveguide with said side wall extending alongthe waveguide so as to overlap the open end thereof. 2. An antenna feedas claimed in claim 1 with additionally, a second cap similar to saidfirst mentioned cap but of larger dimensions than said first cap, andmeans securing said second cap to said first cap so as to surround saidfirst cap.

3. An antenna feed as claimed in claim 2 wherein said last named meansincludes a conductive spacer joining said first and second caps at theapices of their respective conical bases.

4. An antenna feed for controlling the emission of energy from the endof a waveguide, comprising a cap including a conical base and a sidewall joined to said base, said side wall being in the shape of atruncated cone and being joined to said base at the smaller periphery ofsaid side wall so as to enclose the apex of said conical base, a soliddielectric material filling said cap, said cap being dimensioned so thatthe cavity enclosed by said cap is resonant at the frequency of theenergy transmitted by the waveguide, and means supporting said cap overthe open end of the waveguide so that said open end extends into thecavity of said cap with the apex of said base thereof spaced axiallyfrom said open end.

5. An antenna feed as claimed in claim 4 with additionally, a second capsimilar to said first mentioned cap but of larger dimensions than saidfirst mentioned cap and means supporting said second cap so as tocontain said first mentioned cap, there being sufiicient spacing betweenthe outer surface of said first mentioned cap and the inner surface ofsaid second cap to provide a second cavity therebetween resonant at thefrequency of the energy transmitted by the waveguide.

6. An antenna feed as claimed in claim 5 wherein said first mentionedcap and said second cap are aligned coaxially with the waveguide, andsaid means supporting said second cap includes a conductive spacerplaced eccentrically to the axis of the waveguide.

7. An antenna feed as claimed in claim 6 with additionally, a soliddielectric material filling said second cavity between said firstmentioned cap and said second cap.

8. An antenna feed for controlling the emission of energy from the endof a waveguide, comprising, a first cap having an end portion and a sidewall, means securing said first cap spaced from the end of the waveguideand perpendicularly to the axis thereof with said side wall extendingalong said waveguide so as to overlap the end of said waveguide, asecond cap similar to said first cap but of larger dimensions, and meansfor securing said second cap spaced from and surrounding said first cap.

9. An antenna feed for controlling the emission of energy from the endof a waveguide, comprising, a first cap having a conical base and a sidewall, said wall having a length greater than one-half the wavelength inair of the energy transmitted by the waveguide and being joined to saidbase so as to enclose the apex thereof, means for supporting said firstcap in spaced relationship to the open end of the waveguide and withsaid side wall extending along the waveguide so as to overlap the end ofsaid waveguide, a second cap similar to said first cap but of largerdimensions, and means securing said second cap in envelopingrelationship with said first cap.

10. An antenna feed as claimed in claim 9, wherein the side wall of saidsecond cap is substantially equal in length to one wavelength in air ofthe energy transmitted by the waveguide and the spacing between the openend of the waveguide and the apex of the base of said first cap isbetween 0.4x and 0.5). of the wavelength in air of the energytransmitted by the waveguide.

References Cited in the file of this patent UNITED STATES PATENTS2,482,158 Cutler Sept. 20, 1949 2,566,900 McArthur Sept. 21, 19512,750,588 Hennessey June 12, 1956

